Generally speaking, the basic structure of a primary alkaline cell includes a positive electrode (“cathode”) that receives electrons from a negative electrode (“anode”) that releases electrons. The cathode is joined to a positive terminal of the battery by a drawn steel container. The negative electrode is typically a high-surface area metal such as zinc. The anode metal is provided in an electrolyte solution, such as potassium hydroxide dissolved in water, the electrolyte solution being the ion transfer medium between the anode and cathode. A separator which passes ions, but not electrons, is placed between the electrodes.
It is common in the art to provide a gelled anode wherein the gelled portion includes the anode metal, provided as a powder, an aqueous alkaline electrolyte, and a gelling agent for fixing the anode metal and electrolyte in the gel state. Organic and inorganic inhibitors can be added to the anode to suppress gas generation and silicate inhibitors can be added to the anode to suppress electrical shorting through the separator. Conventional gelling agents include carboxymethylcellulose, cross-linking-type branched polyacrylic acid, natural gum, or the like. A typical anode metal is a zinc alloy powder.
Superabsorbent materials can be used in alkaline cells as electrolyte reservoir sites and can also be used to improve zinc particle-to-particle contact. Conventional materials used as superabsorbent materials are polyacrylics that have an irregular shape and a large particle size distribution. The irregular shape and the large particle size distribution of the superabsorbent materials used in conventional alkaline cells can result in inadequate electrolyte availability at some sites within the anode assembly and at the discharge fronts, leading to incomplete discharge at these sites. Further, superabsorbent materials of large sizes can form agglomerates, which can add to the accumulation of discharge reaction products and adversely impact zinc particle-to-particle contact, making some zinc inaccessible.
During discharge, such as by high current drawn from a battery, water is consumed at cathode reaction sites as follows:2MnO2+2H2O+2e−→2MnOOH+2OH−Simultaneously, at the anode electrode, hydroxide ions are needed to sustain the following anodic cell reaction:Zn+4OH−→Zn(OH)4−2+2e The resulting net cell reaction is therefore:Zn+2MnO2+2H2O→ZnO+2MnOOHThe net cell reaction suggests that a continuous supply of water from the electrolyte is necessary to sustain the net cell reaction and avoid early failure due to electrolyte starvation.
The industry has seen an increased demand for use of such cells in high-current environments, including portable audio equipment, cameras and flashes. Cells are likely to discharge in such applications faster than in previous applications. These cells are more sensitive to erratic internal resistance and require the availability of adequate amounts of electrolyte.
Accordingly, there remains a need in the art for an alkaline electrochemical cell that has enhanced cell discharge performance.